The present invention relates to a process for the manufacture of amorphous chlorinated polyethylene. More particularly, the present invention relates to an anhydrous process for preparing amorphous chlorinated polyethylene.
Various processes for chlorinating olefin polymers in general and ethylene polymers in particular are known. The processes can generally be distinguished into three types, namely the chlorination of polyethylene "in solution", "in suspension" or "in bulk".
Chlorination of polyethylene in solution is known from U.S. Pat. No. 3,060,164.
Chlorination of polyethylene in an aqueous suspension is known from U.S. Pat. No. 2,592,763.
U.S. Pat. No. 2,890,213 describes the chlorination of polyethylene in a fluidized bed, and British Pat. No. 834,905 describes the chlorination of free-flowing polyethylene, in each case in the absence of a liquid suspension agent or solvent, the so-called "chlorination in bulk."
The chlorinated polyethylenes obtained by these different chlorination methods show differences in properties, even if one starts from the same polyethylene material, and chlorinates to the same chlorine content. This appears to be connected principally with the more or less statistical distribution of the chlorine in and along the chlorinated polyethylene polymer chains, as is explained in French Pat. No. 1,316,044 and in British Pat. Nos. 843,209 and 950,374. Chlorinated polyethylenes with a non-statistical distribution of the chlorine atoms possess a much higher glass transition temperaure than do chlorinated polyethylenes having a statistical chlorine distribution, and are also much more rigid, hard and brittle.
A number of disadvantages attach to each of the aforementioned three methods, chlorination in solution, in suspension, or in bulk.
The process of chlorination in solution is expensive. The solubility of polyethylene in organic solvents which are inert to chlorine is relatively small, thus large quantities of solvent are needed. As a rule, expensive polyhalogen hydrocarbons must be used for this purpose. The chlorinated polyethylene product must also be later freed from solvent and removal of the last traces thereof is difficult. The separation itself is cumbersome and expensive.
At relatively low temperatures, the chlorination of polyethylene in suspension does not proceed in a statistical manner. A kind of block copolymer structure, having both chlorinated and non-chlorinated segments in the molecule, appears to be obtained, unless the chlorination is continued for very long times until high chlorine contents result. Although a variety of chlorinated polyethylene products may be produced by suspension chlorination, a number of disadvantages are inherent in a suspension chlorination process. First, effluent from the process is a relatively weak hydrochloric acid which must be substantially removed from the chlorinated polyethylene product. Removal generally requires use of a material such as caustic soda which results in an undesirable salt-containing effluent. Second, the chlorinated polyethylene products so produced must be dried before packaging. A third disadvantage is corrosion of processing equipment due to presence of aqueous mixture of chlorine and hydrochloric acid.
Usually, however, it is desired to obtain a soft and flexible chlorinated polyethylene with a crystallinity of less than about 2 percent. In other words, the chlorinated polyethylene should be fully or at least generally amorphous.
In French Pat. No. 1,316,044, a two-stage process has been described for obtaining generally statistical chlorination of polyethylene. In the two-stage process, the second stage is carried out at a rather high temperature, above the crystalline melting point of the polyethylene. "Crystalline melting point" as used herein means the temperature at which crystallinity disappears as evidenced by disappearance of double refraction, or characteristic X-ray diffraction lines. Preferably, the disappearance of crystallinity is measured by Differential Scanning Calorimetry.
Chlorination in suspension at a high temperature introduces the disadvantage that, at such high temperatures, and even under pressure, chlorine will dissolve to only a very small degree in the suspension agent (which is generally water). Subsequent processing of the product by centrifuging and drying is also expensive. Moreover, the by-product, hydrogen chloride, generally cannot be usefully recovered.
In British Pat. No. 843,209 the differences between chlorinated polyolefins with a statistical and a nonstatistical chlorine distribution are also discussed. Intermediate forms thereof are called hybrid polymers. The hybrid polymers are prepared according to British Pat. No. 843,209 by carrying out the chlorination partly in suspension and partly in solution. However, such a process is very cumbersome. The polyethylene chlorinated in suspension must first be separated from the suspension, and then subsequently dissolved. Only then can the second solution stage be carried out.
Chlorination in bulk, as described in U.S. Pat. No. 2,890,213 and British Pat. No. 834,905, can be carried out at rather high temperatures, with the chlorination reaction proceeding rather rapidly. However, a disadvantage of this method is that the chlorination generally proceeds too fast during the initial phase. Since chlorination is an exothermic reaction, when finely divided polyethylene is chlorinated without a solvent or suspension agent local overheating of the polymer occurs very easily. As a result, reaction speed is locally increased in those places. Local development of heat can in turn cause melting, agglomeration and lump formation, as well as decomposition and even burning of the polymer. These circumstances are also attended by the development of an extremely undesirable discoloration in the product.
Chlorinations partly at temperatures below the crystalline melting point and partly above the crystalline melting point are not only known from French Pat. No. 1,316,044 and British Pat. No. 843,209, but also from U.S. Pat. Nos. 2,398,803; 2,920,064; and British Pat. Nos. 1,073,504 and 1,036,360.
When chlorinating in bulk, it is also difficult to avoid sintering of the ethylene polymer particles to some degree. This forms agglomerates which, in turn, cannot be homogeneously chlorinated. The ethylene polymer also becomes discolored due to hot spots. To avoid such discoloration, it has been proposed that chlorination be carried out at low temperatures to minimize the occurrence of hot spots. The temperatures at which such chlorination is to be carried out, at 60.degree. to 70.degree. Centigrade (.degree.C.), are considerably below the crystalline melting point. Low temperature chlorination must be continued until a certain minimum chlorine content is achieved. Only then can the reaction temperature be increased to a temperature above the crystalline melting point of the polyethylene. If the initial low temperature chlorination has been inadequate, further chlorination at the higher temperature will still lead to development of hot spots, agglomerations and attendant discoloration. However, chlorination at temperatures below the crystalline melting point of the polyethylene for an extended period of time generally results in production of large amounts of hard and brittle chlorinated polyethylene.
To eliminate such objections and disadvantages of the chlorination process in bulk, it has also been proposed to mix the ethylene polymer with inorganic salts, which are then to be washed out after the chlorination. Washing out and recovering, and/or discharging those salts, is, however, expensive, and renders this process economically unattactive.
An additional proposal to eliminate such disadvantages centers around selection of the polymer to be chlorinated. In U.S. Pat. No. 4,029,862, the olefin polymer to be chlorinated is characterized by having a flowability measured as angle of repose in the range of from about 24.degree. to about 28.degree., a bulk density in the range of from about 25 to about 35 pounds per cubic foot and a surface area in the range of from about 2 to about 4 square meters per gram. The polymer is chlorinated to an extent of up to about 65 percent by weight chlorine using reaction temperatures not exceeding about 80.degree. C. to provide a chlorine content of up to about 30 percent by weight and thereafter maintaining the reaction temperature from about 60.degree. C. to about 100.degree. C. In U.S. Pat. No. 4,197,386 an ethylene polymer starting material has the following combination of characteristics: (i) at most 5 mole percent of at least one .alpha.-alkene comonomer having from 3 to 8 carbon atoms, particularly propene or butene, (ii) having a density of 0.930-0.970, (iii) a melt index of at most about 5, (iv) a particle size distribution of between about 50 to 2000 microns, (v) a porosity of at most about 0.15 cubic centimeters per gram, (vi) a surface area determined in accordance with the method described in J. Am. Chem. Soc. 60 309 (1938) by Braunauer, Emmet and Teller of at most about 1 square meter per gram and (vii) a wax content of at most about 1 percent by weight.
In view of the foregoing, it is an object of the present invention to provide a process for preparing amorphous chlorinated polyethylenes which are generally soft and flexible yet free of discoloration.
It is also an object of the present invention to provide a non-crystalline, amorphous chlorinated polyethylene having satisfactory properties such as good flexibility and high elongation. Such chlorinated polyethylenes are suitable for use as impact modifiers in polyvinylchloride.